1. Field of the Invention
The invention relates to Lorentz Force magnetometers or magnetic field sensors that use mechanical resonators to enhance sensitivity including a simple, small, lightweight, low-cost, and low-power-consumption sensor that utilizes a resonant string and the Lorentz Force to measure multiple vector magnetic fields.
2. Description of the Related Art
There is increasing interest in the development of miniature magnetometers for mapping magnetic fields found in space, industrial, environmental, and biomedical applications. The trend is constantly toward smaller size, lower power consumption, and lower cost for similar performance. Toward this end, recent developments have included the use of piezoresistive cantilevers (originally developed for atomic force and scanning tunneling microscopy) and xcexc magnetometers (based on electron tunneling effects).
The problem with the above devices is that they require, at least in some stages of their assembly, extensive and intricate processing. Furthermore, their sensitivities, defined as the minimum detectable field change, are generally in the range of 1 xcexcT to 1 xcexcT. Therefore, there remains a need for magnetometers with increased sensitivity in which size, power and cost are reduced.
Devices such as the pendulum clock or, more recently, quartz crystal resonator controlled watches use mechanical resonators to enhance detection. In both examples, the accuracy is directly linked to the quality factor or xe2x80x9cQxe2x80x9d of the resonator.
A new type of mechanical resonator magnetometer based on excitation of a resonant bar configured in a xylophone geometry with supports at the nodes of the first transverse node is described in U.S. Pat. No. 5,959,452, issued Sep. 28, 1999, by Givens et al which is incorporated herein by reference. The xylophone magnetometer measures the vectorial component of the magnetic field which lies in the plane of the xylophone and is perpendicular to its major axis.
The response of the xylophone magnetometer was linear to a low frequency magnetic field over 7 decades of range and had a noise floor below 1 nanotesla. The high sensitivity of this sensor was based in part on the high resonant 0(≈10,000) of the xylophone resonator (0=f0/xcex94f where f0 is the resonance frequency and xcex94f is the full width at half maximum of the resonance response).
While the xylophone magnetometer was a significant improvement, there remains a need for a magnetometer that can measure multiple vector magnetic fields and whose resonant frequency can be easily and dynamically varied. Ease of manufacture particularly in arrays for use in biomedical applications, particularly in catheters, is another goal.
The invention, a new type of mechanical resonator magnetometer utilizing a resonant string and based on the response to a Lorentz Force, is a significant improvement over the xylophone magnetometer disclosed in U.S. Pat. No. 5,959,452. The invention can measure multiple vector magnetic fields, can rapidly change resonant frequency and can be easily manufactured in arrays. All of these new features allow the invention to be useful in certain application areas including medical applications as discussed below.
The mechanical resonator magnetometer of the invention comprises an electrically conducting string or an insulating fiber coated with an electrically conducting material wherein the fiber may be light conducting, and means for supporting the string or fiber in tension at two locations. When a current is inserted in the string or fiber and the magnetometer placed in a magnetic field, the resulting Lorentz Force will cause the string or fiber to deflect along multiple axes that can be detected. Tension of the string or fiber can be varied using, e.g., piezo or MEMS elements. Detection of the light conducting fiber embodiment of the invention with high sensitivity and in a compact manner may be had by forming an aperture in the electrically conducting material coating the fiber and detecting the emitted light.
In the vibrating string magnetometer of the invention, the resonant structure is the string itself. Familiar devices based on the resonant string are such musical instruments as the violin and guitar. For these devices the resonant frequency is determined by the string mass per unit length, string length and tension applied to the string. The quality factor is more difficult to determine but is broadly related to energy losses due to internal friction, to the string supports, and to the air.
An important issue in maintaining a high quality factor is keeping losses low. This directly relates to the issue of detection of the motion of a string which carries a current in the presence of a perpendicular magnetic field. The Lorentz Force is given by FL=Jxc3x97B where J is the vector current and B the vector magnetic field. If the direction of the string is along the y-direction, then J=Jy and the interacting fields, Bz and Bx, produce forces (and motion) along the x and z axes respectively. By measurement of the motions, it is possible to measure the magnetic fields Bz and Bx simultaneously.